1. Field of the Invention
This invention relates to down converters for semi-parabolic antennas for use in microwave systems such as multichannel multipoint distribution systems (MMDS) (also termed "wireless cable") and, in particular, to integrating the feed with the down converter into an integrated structure.
2. Statement of the Problem
Wireless cable is competing with cable to offer programming to the public. Wireless cable utilizes a semi-parabolic reflector that is mounted on the rooftop of a consumer's house. Multiple channels are delivered to this semi-parabolic reflector from a remote location. Conventional microwave semiparabolic antenna systems utilize an antenna with a separate down converter mounted on the backside with coaxial cable and connectors interconnecting the two elements.
A need exists to integrate the separate down converter into the semi-parabolic reflector in order to reduce wind load of the overall antenna and down converter receiving system, to provide a system that can be installed by the consumer with a minimum of installation time, to eliminate the need for a separate down converter housing (thus reducing the material and labor costs of the system), to eliminate the coaxial cable and connectors interconnecting the antenna and the down converter in order to reduce the signal loss between the antenna and the down converter (thereby improving the overall system signal-to-noise ratio at a significantly lower materials cost), and to improve the impedance matching between the antenna and the down converter, which allows for an optimum signal transfer.
A further need exists to provide a rigid mechanical and electrical connection between the feed and the down converter printed circuit boards without using an intervening cable or the like. This need exists because of the environmental stresses such as vibration from wind and expansion from wide temperature swings.
3. Results of a Patentability Search
A patentability search was conducted on the teachings of the present invention with respect to a solution to the above problem. The following patents were uncovered in this search:
______________________________________ U.S. Pat. No. Inventor Issue Date ______________________________________ 3,162,858 Cutler 12-22-64 4,290,068 Bogner 9-15-81 1,149,931 Thomas (Canada) 7-12-83 ______________________________________
The 1964 patent to Cutler sets forth a ring focus antenna feed for a parabolic dish antenna. Cutler discloses a feed connected to a circular waveguide which in turn is connected to an amplifier. The amplifier and the circular waveguide are contained within a supporting pedestal. The amplifier is then interconnected through a second circular waveguide to the transceiver. The second circular waveguide and the transceiver are located behind the parabolic dish. The amplifier and the first circular waveguide are located in the pedestal that supports the feed.
The 1981 patent to Bogner sets forth a multipoint distribution system (MDS) disk-on-rod-type antenna. To the rear of the spaced-apart disk is located a waveguide that includes a cylindrical container having a closed back and sidewalls with an open mouth. Bogner incorporates the down converter into the waveguide in order to reduce wind load and to reduce transmission loss. In implementing the down converter into the waveguide, Bogner requires that it must be contained in less than fifty percent of the volume of the waveguide cavity. Bogner houses his down converter in a separate metal structure within the waveguide. Because the down converter is located in a waveguide, the waveguide must have precise dimensions based on the walvelength of the signal being sensed. Bogner requires that the metal structure containing the down converter electronics must not touch the sidewalls of the waveguide since to do so would change the waveguide and wavelength. The Bogner approach is critically designed for a frequency of 2153 MHz.
The 1983 Canadian patent to Thomas also relates to an MDS antenna suitable for receiving a single microwave transmission in the 2.15-gigahertz range. Thomas utilizes a semi-parabolic antenna having its feed supported by a hollow tubular support in which Thomas integrates the down converter or a portion of the down converter. Thomas connects the feed elements through a matching network to a coaxial cable that interconnects the matching network to a mixer located within the tubular support. The mixer carries the input RF signal from the feed elements to a signal from the local oscillator, and the difference of the input signal (2154.75 MHz) and the local oscillator frequency (2216.0 MHz) is delivered into a set of IF amplifiers whose output is connected to a standard coax cable. The circuit board carrying the electronics contained within the tubular support is grounded to the tubular support by means of an arcuate metal conductor. The conductor electrically connects the printed overlay of the board with the inside metallic surfaces of the support arm. The outer sheath of the coaxial cable that interconnects the mixer with the matching network is also connected to the copper overlay of the circuit board. Likewise, the outer sheath of the coaxial cable interconnecting the outputs of the IF amplifiers and the customer's television set is also grounded to the copper overlay.
Of the three patents found in the search, only the Thomas patent is pertinent. The Bogner approach utilizes a critically designed waveguide having disposed within the waveguide the down converter electronics. The package for the down converter electronics is critically designed and is electrically insulated from the waveguide. Cutler places the amplifier within the feed support but requires a waveguide to extend the signal from the feed to the amplifier. Thomas, like the present invention, utilizes a semi-parabolic antenna and places the down converter or a portion thereof into the tubular support arm that supports the feed. Thomas, while solving the problem with respect to minimizing wind load and reducing the number of separate parts and components, does not provide a system for providing MMDS programming. Hence, a need still exists when implementing the down converter into the tubular support arm to provide RF filtering in order to minimize IF and image type interference, to provide circuit shielding to prevent unwanted signals from radiating past input filtering, to provide multiple grounding connections between the circuit board and the inside of the support boom to maintain the integrity of RF filtering and shielding, to utilize an RF amplifier stage to establish low noise performance, to provide broad band mixing performance with high signal handling characteristics, to reference a stabilized oscillator, and to provide an integrated down converter/antenna system that is compatible with encryption/addressable systems. The Thomas approach was only concerned with processing a single microwave TV channel and was not concerned with the interference potential of other channels in an MMDS configuration. Furthermore, Thomas did not fully eliminate the coaxial cable interconnections.
4. Solution to the Problem
The present invention provides a solution to the above stated problem, a solution that is not met by the above three patents.
The integrated antenna/down converter of the present invention provides a one-piece concept to MDS receive antennas. By integrating all the necessary components in the antenna, installation times are kept to a minimum and there is a reduction in inventory parts used by the MMDS operator. The integrated antenna/down converter unit of the present invention can be packaged for consumer installation to allow for more cost savings by the system operator. All necessary cables and connectors are provided for consumer installation. The semi-parabolic reflector of the present invention mounts the down converter in the feed support boom.
The integrated antenna/down converter of the present invention also provides significant RF filtering so as to minimize IF and image type interference. This is accomplished by providing circuit shielding at the input from the feed that prevents signals from radiating past the input filtering and at the output. Multiple grounding connections exist between the circuit board and the inside of the support boom at the input and output ends. Furthermore, an RF amplifier stage is provided to obtain low noise performance. Broad band mixing performance with high signal handling characteristics are also provided. The circuit of the present invention utilizes a stabilized reference oscillator physically isolated from the main portion of the electronics which also minimizes signal interference. The integrated down converter/antenna system of the present invention is compatible with encryption/addressable systems. Finally, the circuit of the present invention fully eliminates all coaxial cable interconnections between the down converter and the driven feed elements and provides mechanical structural reinforcement between the feed and the down converter printed circuit board and when the feed is mounted on a printed circuit board. Solid electrical connection between the output of the feed and the input of the down converter is also provided.